Cytoskeleton organization and lysosome secretion are critical for osteoclast activation and function. However, the molecular mechanisms regulating these processes are poorly understood. The goal of this grant application is to elucidate the mechanisms by which the cytoskeletal and lysosomal proteins regulate osteoclasts. PLEKHM1, a newly identified protein mutated in human and rat osteopetrosis, is critically involved in osteoclast lysosome trafficking and secretion. Furthermore, Plekhm1 was found to bind to LIS1, a key regulator of microtubule dynamic in eukaryotic cells. LIS1 interacts with dynein/dynactin, a motor complex that regulates microtubule dynamic and transportation. LIS1 also binds to the catalytic 1-subunit of PAF-AH (platelet-activating factor (PAF) acetylhydrolase) 1b complex, which inactivates PAF, a lipid messenger functional important for osteoclast survival and activities. LIS1 has also been shown to regulate cdc42, a small GTPase that is required for bone homeostasis in mice by modulating M-CSF and RANKL signaling. More importantly, LIS1-flox;LysM-Cre mice, in which LIS1 is specifically deleted in myeloid cells, have increased bone mass and impaired osteoclast formation and bone resorption. These data led to the hypothesis that Plekhm1 is essential for skeleton homeostasis and Plekhm1/LIS1 interaction is critical for lysosome secretion and bone resorption. LIS1 regulates osteoclast formation and function through its modulation of dynein function and M-CSF/RANKL signaling pathways via PAF and/or cdc42. To test these hypotheses, genetically modified mice and osteoclasts derived from these mice will be used to: a) determine the role of Plekhm1 in osteoclast function and identify the mechanisms mediating Plekhm1/LIS1 interaction (Aim 1). b) determine whether LIS1 regulates osteoclast function and dissect the mechanisms by which LIS1 regulates microtubule organization and Cathepsin K secretion in osteoclasts (Aim 2). c) determine whether LIS1 regulates osteoclastogenesis and define the molecular mechanisms by which LIS1 modulates M-CSF and RANKL signaling pathways (Aim 3).